What Causes Delayed Periods And Absence Of Menstrual Symptoms?

Hi All,
Me n my girlfriend are having regular sex during weekends when we meet..Almost 5-6 times every weekend...now her periods were due on 4th december, however she missed her dates and last weekend we had urine pregnancy test but results were negative...
she usually get late periods but always this time she is not even feeling any abdominal pain which she usually feels..
Please suggest.